The main significance of this study is to show how a host mounts an immune response to reject a tumor and to explain why some individuals are unable to respond. Now we are trying to determine the mechanism of how and why FeSV adapts to heterospecies cells, and in this process become oncogenic for the unrelated species. Sheep are being used as the model species because the immune response is well studied in this animal. We have demonstrated that regional lymphatics play a role in defense against tumor growth and have shown there also is a similar defense in peripheral blood. The defense mechanisms are complex and they act in a manner similar to rejection of an organ transplant. The "blocking" or "interfering" effect of the immune response in allowing tumors to grow may be due to soluble substances in the sera. This aspect of tumor growth will be investigated in detail. It is anticipated that this information can be extrapolated to man, to better understand why some people get tumors and ultimately lead to better methods of prevention and control. In addition, during the third year, we will investigate new methods of immunotherapy.